1. Field of the Invention
This invention relates to a hydraulic jarring tool for earth boring applications and, more particularly, to such a jarring tool having a hydraulic fluid by-pass valve permitting telescopic extension of the tool without a jarring impact being delivered thereby.
2. Description of the Prior Art
Hydraulic jarring tools are tools commonly included in a drill string for drilling oil and gas wells and have a primary function of permitting an up-jarring or sudden impact effect when the drill string is being lifted by the draw works to place the drill string under tension when it is stuck in the hole. The up-jarring impact of the tool is utilized to free the stuck drill string.
To accomplish the up-jarring effect, it is typical for the tool to have two telescopically moveable concentric cylinders defining therebetween an annular sealed chamber containing a hydraulic fluid. The chamber is partitioned by an annular sleeve valve disposed therebetween into an upper hydraulic chamber and a lower hydraulic chamber. A valve in a fluid passage in the sleeve valve permits the fluid to flow from the upper chamber into the lower chamber at a relatively uniform, yet restricted flow rate under pressure from a decreasing volume of the upper chamber as the cylinders are urged to telescope outwardly.
Typically the portion of the drill string which is stuck in the borehole would be below the tool (i.e. at the drill bit) so that lifting the drill string by the draw works, in addition to extending the jarring tool, places the upper portion of the drill string or that portion above the stuck apparatus in tension. By pulling upwardly on the drill string with sufficient force and speed that the hydraulic fluid in the upper chamber cannot flow through the valve at a rate to permit the tool to extend telescopically at the same rate, the drill string is stretched, much like an extended spring. The continued telescopic extension of the tool permits the cylinders to attain a predetermined extended position whereat a fluid path around the sleeve valve is available, permitting a free extended movement of the cylinders therefrom. This permits an immediate release of the stretched drill string which suddenly retracts to cause opposed anvil surfaces in the concentric cylinders to contact under this spring force and provide an up-jarring impact to unstick the drill string.
However, during drilling operations there are often occasions when a component of the drill string becomes stuck in situations that require a downwardly delivered blow or bumping action (as opposed to the above-described up-jarring impact) to free it. On such occasions, an upwardly delivered blow may further exacerbate the stuck condition.
One method of providing a down-jarring or bumping effect on a stuck drill string is to gradually pull upwardly on the string, again to a stretched position, and then suddenly release the brake on the draw works supporting the string, permitting the drill string to retract. After the drill string has retracted a certain distance, as monitored on the drill table, the draw works' brake is engaged to stop the downward motion of the top of the string, thereby imparting a downward impact to the stuck portion of the string. With a jarring tool in the string an additional stroke distance is available for the upper portion of the string to travel to gain velocity and provide greater impact by the amount the two cylinders of the jarring tool have been extended. However, if a jarring tool, such as above-described, is included in the drill string, the upward force to lift the drill string and extend the tool also causes the jarring tool to respond (i.e. fire) as described above, with an upward jarring effect that may be detrimental to the stuck component, or drive it further into a wedged or stuck position. This may ultimately undo the result of any downward bumping being attempted.